The overall objective is to investigate the involvement of transfer RNA in the regulation of amino acid biosynthesis and to determine the functions of tRNA modifications in these controls. To fulfill this objective the following approaches will be applied. 1) The information already obtained from studies of biosynthetic pathways and tRNA modification mutants will be extended to investigations of new pathways. The results of these physiological experiments will be evaluated to determine whether a role for tRNA exists in repression and/or attenuation controls for these pathways. 2) New mutants of Salmonella typhimurium having either altered tRNA moddifications or structures will be isolated. These mutants will be characterized genetically and physiologically to examine the effects of these changes on cell growth and regulation. 3) The biochemical consequences of these mutations will be determined by purifying the tRNA and identifying the changes in the nucleotide composition. The effects of the incomplete modifications and structural changes on the aminoacylation reaction and in vitro translation process will be measured.